Such heat exchangers are used especially in motor vehicles having internal combustion engines, either for engine cooling purposes or for heating the cabin of the vehicle. An engine coolant fluid flows through the tubes in the bundle, while air circulates around the tubes. In certain cases, the tubes have a non-circular cross section, and in particular a flattened cross section, generally oval, eliptical or oblong, such as to reduce the loss of kinetic energy in the air flow passing through the heat exchanger and to optimise, to some extent, its thermal performance.
In the known heat exchangers of this type the same number of holes is provided in the collecting plate as there are tubes in the bundle, so that each tube end portion is received individually in a separate hole in the collecting plate and is sealingly joined to the collecting plate, and in particular to the above mentioned lip surrounding the relevant hole, if such a lip is provided. The collecting plate (which is also known as a perforated plate or tube sheet) is generally made in the form of a metallic plate in which are formed both the holes and the corresponding lips. The seal between each end portion of a tube and the corresponding lip of the collecting plate is then achieved either by interposing a compressible sealing element or by direct brazing, with the tube then being provided with a metallic coating having a low melting point. Since these heat exchangers comprise a plurality of rows of tubes, the collecting plate must also include a plurality of rows of holes which are separated from each other by a given pitch distance within any given row, and by a further given pitch distance as between one row of holes and the next. There thus exists between two consecutive holes in the collecting plate a thickness of material which, for reasons of mechanical strength of the collecting plate, must not be less than a certain minimum value. This minimum value is also essential in order to enable any lips to be formed around the holes in the collecting plate.
As a consequence, it has not hitherto been possible to realise the potential for optimisation of the dimensions of the tubes and their pitches, and therefore the optimisation also the thermal performance of the heat exchanger, for a given set of dimensions of collecting plate.